Method and device for measuring the diameter of a peripheral rod in a fuel assembly of a nuclear reactor

ABSTRACT

The diameter of a segment of a peripheral rod in a fuel assembly is measured between two successive spacer grids. A measuring tool is positioned at a level situated between the two successive spacer grids, the tool comprising two measuring fingers that are free to move relative to each other in a first horizontal direction and that are urged towards each other by a resilient return arrangement, and that extend in a second horizontal direction perpendicular to the first horizontal direction. The measuring fingers are moved in the second horizontal direction towards a rod so as to put two contact pieces of the measuring fingers into contact with two zones of the rod segment. The measuring fingers are moved in the vertical axial direction of the rod along its segment situated between the two spacer grids and an electromagnetic sensor is used to measure the distance between the measuring fingers in the first horizontal direction while the measuring fingers are being moved axially.

FIELD OF THE INVENTION

The invention relates to a method and to a device for measuring thediameter of a peripheral rod of a fuel assembly of a nuclear reactorthat is cooled with light water, and in particular a fuel assembly for anuclear reactor that is cooled with pressurized water.

BACKGROUND INFORMATION

The fuel assemblies for reactors cooled with light water and inparticular the fuel assemblies for reactors cooled with pressurizedwater generally comprise a bundle of fuel rods each made up of a stackof pellets of fuel material inside tubular cladding, and a framework forsupporting the fuel rods, the framework comprising spacer grids that arespaced apart along the rods of the bundle, guide tubes that occupycertain rod positions inside the bundle, and two end nozzles.

Inside the bundle, the fuel rods are supported in parallel dispositionsby the spacer grids which are constituted by respective square-mesharrays of square cells, each of which has a fuel rod engaged therein, orpossibly a guide tube in certain positions inside the array.

The spacer grids and the end nozzles which are fixed to the ends of theguide tubes at opposite ends of the bundle of rods are square in shapeso that the overall shape of the fuel assembly bundle is that of a rightprism of square section, i.e. that of a rectangular parallelepiped.

The fuel rods of the bundle that are located in the outside faces of thefuel assembly that are perpendicular in pairs constitute the peripheralrods of the fuel assembly, and they

Inside the cells of the spacer grids, the fuel rods are held by springsbearing against rigid abutments, thus making it possible both to supportthe rods in transverse planes in a regular array and also in the axiallongitudinal direction of the bundle.

In a nuclear reactor in operation, the fuel assemblies are subjected toirradiation which leads to certain modifications in the structure andsize of the elements constituting the fuel assembly.

In particular, the pellets of fuel material in the rods can swell to acertain extent while the nuclear reactor is in operation. This resultsin the tubular cladding of the rods deforming and in an increase in thediameter of the rods.

In order to know how the fuel behaves in the nuclear reactor and inorder to plan maintenance operations on the fuel assemblies, it can beimportant, after the fuel assembly has been in use for a certain lengthof time in the core of the nuclear reactor, to measure the diameter ofthe rods, at least in certain zones of the rods.

It is advantageous to be able to perform such measurements withouttaking the fuel assembly apart, but under such circumstances, diametermeasurements are restricted to the peripheral rods in the fuel assemblythat are accessible in the side faces of the assembly.

In addition, while the nuclear reactor is in operation, the reactorcooling water circulates a high speed in contact with the rods held inthe framework by springs and dimples in the cells of the spacer grids.

As a result, the rods are caused to vibrate and are therefore subject toa certain amount of wear where they come into contact with the dimplesinside the spacer grids.

It is necessary to be able to measure the wear in those segments of therods that lie inside the spacer grids after the fuel assembly has beenin use for a certain length of time in the nuclear reactor.

It is desirable to be able to perform these measurements without havingto take the fuel assembly apart.

Fuel assemblies taken out from the nuclear reactor, after the reactorhas stopped and cooled, are generally deposited in a deactivation poolsituated in a fuel building, close to the building that houses thenuclear reactor.

Fuel assemblies are moved within the fuel pool by using a handling andhoist tool carried by a fuel pit bridge that moves over the fuel pool.In addition, an elevator which is fixed to a wall of the pool serves tomove the fuel assemblies in the vertical direction.

The unloading of fuel assemblies that is performed after the nuclearreactor has cooled makes use of a loading machine which travels over areactor pool into which the nuclear reactor vessel containing the fuelassemblies opens out, the vessel and the pool being filled with waterwhile the fuel assemblies are being handled.

The fuel assemblies can be measured either in the fuel pool or in thenuclear reactor pool, depending on how the operations of loading andunloading the fuel assemblies are performed.

Nevertheless, a method has not been known in the past enabling thediameter of peripheral rods to be measured with very high measurementaccuracy in a fuel assembly along those segments of the rod that extendbetween two successive spacer grids or along those segments of the rodsthat are received inside the spacer grids and that are subjected to wearin the reactor in operation, without it being necessary to take the fuelassembly apart.

SUMMARY

The object of the invention is thus to propose a method of measuring thediameter of a peripheral rod of a fuel assembly of a nuclear reactorcooled by light water, the fuel assembly comprising a bundle of fuelrods and a framework for supporting the fuel rods, the frameworkcomprising spacer grids distributed in an axial longitudinal directionof the bundle of rods, guide tubes disposed in certain positions of therods inside the bundle, and end nozzles, the measurement being performedunderwater in a pool in which the fuel assembly is placed so that itslongitudinal axis extends vertically, and being performed between twosuccessive spacer grids along a longitudinal segment of a rod situatedat the periphery of the fuel assembly, which method makes it possible toperform highly accurate measurements remotely and without taking thefuel assembly apart.

For this purpose, the method comprises the steps of:

-   -   placing a measuring tool at a level situated between two        successive spacer grids, the tool having two measuring fingers        with freedom to move relative to each other in a first        horizontal direction, the fingers being urged towards each other        by a resilient return arrangement and extending in a second        horizontal direction perpendicular to the first horizontal        direction;    -   moving the measuring fingers in the second horizontal direction        towards the rod so as to put two contact pieces into contact        with two zones of the segment of rod occupying diametrically        opposite generator lines, the contact pieces being secured to        respective ones of the measuring fingers;    -   moving the two measuring fingers whose contact pieces are        pressed against the rod in two zones situated on diametrically        opposite generator lines, so that they travel in the vertical        axial direction of the rod along the segment situated between        the two spacer grids; and    -   using an electromagnetic sensor to measure the distance between        the contact pieces of the measuring fingers in the first        horizontal direction perpendicular to the axial displacement of        the measuring fingers.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to make the invention well understood, there follows adescription given by way of example and with reference to theaccompanying figures, and relating to a measuring device in accordancewith the invention and to the use of the measuring method in accordancewith the invention both for measuring the diameter of a longitudinalsegment of a peripheral fuel rod of a fuel assembly for a pressurizedwater nuclear reactor, said segment extending between two spacer grids,and for measuring the wear of a rod in a support zone inside a spacergrid.

FIG. 1 is a diagrammatic elevation view of a fuel assembly and of a toolfor measuring the diameter of the peripheral rods in the fuel assembly.

FIG. 2 is a plan view looking along arrow 2 of FIG. 1 illustrating aportion of the measuring tool.

FIG. 3 is an overall view of a measuring installation enabling themethod of the invention to be implemented.

FIG. 4 is a diagrammatic side elevation view of a support segment of arod in a spacer grid, during an operation of inspecting wear byimplementing the method of the invention.

FIG. 5 is a plan view partially in cross-section of a rod and of themeasuring device.

DETAILED DESCRIPTION

FIG. 1 illustrates a portion of a fuel assembly 1 for a pressurizedwater cooled reactor, the assembly being partially engaged in a fuelelevator 7 of the deactivation pool of a nuclear power station.

The fuel assembly 1 comprises a bundle of rods 2 and a frameworkcomprising spacer grids 3 for supporting the rods 2, guide tubes 4, andnozzles fixed to the end portions of the guide tubes 4 at opposite endsof the bundle of rods 2.

In FIG. 1, the top end nozzle 5 of the fuel assembly is illustrated withthe fuel assembly being suspended via said nozzle from a sling 6 of ahandling device supported by a pit bridge above the top level of thepool.

The fuel assembly which is fully immersed in the water of the pool isdisposed with its longitudinal axis parallel to the direction of thefuel rods 2 in the bundle, in a vertical direction, and is inserted inpart into the elevator 7 via the top end of the elevator which may bemoved vertically, as represented diagrammatically by double-headed arrow7′.

On the top portion of the elevator 2 having an opening for passing thefuel assembly, there is fixed an adapter ring 8 having a support 9mounted thereon, which support is preferred to as S-support, and in turnit carries an X-Y displacement table 10.

The X-Y displacement table 10 comprises a bottom table 10 a mounted tomove on the support 9 in a first horizontal direction X perpendicular tothe plane of FIG. 1.

A second table or top table 10 b is mounted to move on the table 10 a ina second horizontal direction Y perpendicular to the first horizontaldirection X.

The X-Y displacement tables 10 a and 10 b are motor driven and may bemoved backwards and forwards along the directions X and Y as representeddiagrammatically by double-headed arrows 11 a and 11 b.

The top X-Y displacement table 10 b carries a column 12 having ahorizontal support 13 for the measuring tool 14 mounted to movetherealong in the vertical direction Z.

Motor-driven displacement arrangements are associated with the verticalcolumn 12 and with the support 13 to move the support 13 vertically atlow speed and very accurately, as represented diagrammatically bydouble-headed arrow 15.

The measuring tool 14 comprises a support 14 a carrying two retractableguide pulley wheels 15 a and 15 b mounted to rotate about horizontalaxes and serving to guide the tool 14.

The tool support 14 a is itself floatingly mounted in the verticaldirection Z via guide elements and springs inside a second support 16,itself floatingly mounted on the tool support 13 via two floating tables17 a and 17 b under spring bias and presenting freedom to moverespectively in the direction X and in the direction Y, as illustratedin FIG. 2.

This serves to avoid exerting forces that could damage elements of thefuel assembly 1 while displacing the measuring tool in any of thedirections X, Y, and Z, said tool comprising in particular measuringfingers 18 a and 18 b (FIG. 2).

The measuring fingers 18 a and 18 b of the measuring tool 14 are mountedon the support 14 a with a certain amount of freedom to move in thedirection X, the fingers being urged towards each other in the directionX by springs such as 19.

The fingers 18 a and 18 b extend in the direction Y, and at their endsdirected towards the fuel assembly 1 they have tapering end portions 20a and 20 b which may be engaged on either side of a rod 2 in the fuelassembly, in the two gaps between successive pairs of peripheral rods inthe cross-section of the fuel assembly 1, as shown in FIG. 2.

The measuring fingers 18 a and 18 b which are disposed parallel to eachother in the direction Y with a certain amount of freedom to move in thedirection X are associated at their end portions remote from thetapering portions 20 a and 20 b for engaging in the structure of thefuel assembly 1 with an electromagnetic displacement sensor 21, such asa linear variable displacement transducer (LVDT). Such an LVDT sensorcomprises a sensor body having an electrical coil in the form of asolenoid mounted therein, together with a sensor rod that is secured toa plunger core that is movable inside the solenoid. The coil is fed withelectricity and the induction produced by the coil is modified by movingthe magnetic core inside the solenoid in the direction of the sensorrod. This makes it possible to measure displacements very accurately bymeasuring an electrical current. The sensor body 21 is fixed to one ofthe measuring fingers 18 a and the sensor rod secured to the magneticcore is fixed to the other measuring finger 18 b, the sensor rodextending in the direction X of relative displacement between themeasuring fingers. It is thus possible to measure the displacement ofthe identical fingers very accurately while they are being engaged oneither side of a rod 2 in the fuel assembly, with the diameter of therod being measured via the tapering and curved engagement end portions20 a and 20 b of the fingers.

A first measurement is performed on a reference rod, and subsequentlythe displacement of the fingers is measured while they are being engagedon the rod 2 of the fuel assembly on which a measurement is to beperformed.

The tool support 13 includes a post 22 having mounted thereon a firstvideo camera 23 which is inclined and fitted with a spotlight 23′ and asecond video camera 24 likewise fitted with a spotlight 24′ and facingin a horizontal direction.

The images provided by the cameras are sent to a measuring stationsituated above the level of the deactivation pool in the nuclear powerstation.

The first video camera 23 serves to monitor docking of the engagementportions of the measuring fingers on either side of a rod 2 whosediameter is to be measured, and the second camera 24 has a wider fieldand is provided with aiming arrangement so as to make it possible tofind and aim for the axis of the fuel assembly rod 2 on whichmeasurements are to be performed, so as to make it possible to place anddirect the measuring head of the measuring tool 14.

FIG. 3 is an overall view of an installation for measuring the diameterof rods in a fuel assembly 1, within the deactivation pool of a nuclearpower station.

The measuring installation comprises in particular the measuring stationplaced above the level of the pool and given overall reference 25.

The measuring device 14 as described above is fixed on the support 9which is secured to the adapter ring fixed on the top portion of anelevator in which part of the fuel assembly 1 is engaged. The support 9carries a set of X-Y displacement tables used for moving the measuringtool 14 along two horizontal axes X and Y that are respectively paralleland perpendicular to a front face of the fuel assembly 1 thus enablingthe diameter of a rod in a peripheral row to be measured.

From the measuring station 25 located above the level of the pool, theelevator is caused to move in the vertical direction, the measuring toolis caused to move in the vertical direction along the accuratedisplacement post 12 at low speed, and the displacements of the set oftables 10 in the horizontal directions X and Y are also controlled. Forthis purpose, the support 9 of the elevator is connected by a cable 26to an electronics bay 27. The cable 26 serves to transmit signals to thebay 27 from the video cameras 23 and 24 and to display the fuel assemblyin the zone where the measuring tool 14 is to intervene, and alsodisplay the measuring fingers of the tool 14. The cable 26 also servesto transmit orders to control displacement from the bay 27 to thesupport 9 of the elevator inside the pool of the nuclear power station.

The bay 27 is a display and control bay having screens for displayingthe fuel assembly 1 and the measuring fingers of the tool 14, and alsoan arrangement for controlling the elevator 7 and the X-Y displacementtable 10, and an arrangement for controlling slow displacement of themeasuring tool 14 in association with the post 12.

The measuring station 25 also comprises a bay 28 for controlling andacquiring measurements, which bay is connected via a cable 29 to theLVDT sensor 21 and via a cable 29′ to a temperature probe 29′aassociated with the measuring tool 14.

The bay 28 has a unit 30 for acquiring and processing analog signals,said unit comprising an analog data input card and an analog-to-digitalconverter module.

The analog inputs coming from the unit 30 are constituted by the signalsfrom the LVDT sensor 21 of the measuring tool 14 which are transmittedto the unit 30 via a conditioning module 31, and by the analog signalscoming from the temperature detector 29′ a dipped in the water of thedeactivation pool in the vicinity of the fuel assembly and connected tothe unit 30 via a conditional 32 for conditioning the signals from thetemperature probe.

The digitized signals output by the unit 30 are forwarded to a computer33 which serves to compute and display the measured values of thediameter of the rod 2 in the fuel assembly 1. Measurements are correctedas a function of the temperature as measured by the temperaturedetector.

The signals are also sent to a printer 34 for printing out the measuringvalues and values that are computed on the basis of the data.

There follows a description given with reference to FIGS. 1 to 3 inassociation of an operation for measuring the diameter of a peripheralrod 2 in the fuel assembly 1.

Prior to taking a measurement, the measuring tool 14 is calibrated byplacing the contact portions of its fingers so as to bear against twodiametrically opposite zones of a reference rod, for example in themeasuring station, above the top level of the pool. The correspondingsignals from the electromagnetic sensor 21 are measured and recorded.

The fuel assembly 1 on which the diameter of a peripheral rod 2 is to bemeasured is suspended in a vertical position from a sling of a handlingarrangement carried by the fuel pit bridge.

The fuel assembly is inserted into the top end of an elevator 7 havingan adapter ring 8 fixing the support 9 thereto carrying the set of X-Ydisplacement tables 10 and the measuring tool 14 via the post 12 and thearrangement for moving the measuring tool 14 accurately in the verticaldirection Z.

The fuel assembly is oriented in such a manner that its side face wherethe peripheral rod 2 is to be found on which measurements are to beperformed is directed towards the measuring head 18 of the measuringtool 14 that has the measuring fingers 18 a and 18 b. The face of thefuel assembly facing towards the measuring head 18 is parallel to thehorizontal direction X and perpendicular to the horizontal direction Yin which the X-Y displacement table 10 moves. The elevator 7 carryingthe support 9 is moved vertically up or down as represented by arrow 7′in order to place the measuring tool 14 between two successive spacergrids 3 between which it is desired to measure the diameter of aperipheral fuel rod 2 of the fuel assembly. The distance between twosuccessive spacer grids is about 150 millimeters (mm).

The measuring head 14 is positioned approximately so as to be able toscan the segment of the rod by the measuring fingers 18 a and 18 b ofthe measuring head 18 between the two spacer grids 3, and using the slowdisplacement arrangement associated with the displacement post 12, thisarrangement also serving to provide accurate positioning of themeasuring head 18 in the vertical direction Z.

The docking camera 23 is used to place the measuring head 14 in aposition from which measurements can be started along the segment of thefuel rod 2 between the spacer grids 3.

For this purpose, the set of X-Y displacement tables 10 is used tocenter the measuring head 18 relative to the axis of the fuel rod 2 byperforming movement in the direction X, and then to engage the endportions 20 a and 20 b of the measuring fingers 18 a and 18 b of themeasuring head 18 on either side of the rod 2 by moving in the Ydirection.

The tapering and curved end portions 20 a and 20 b of the measuringfingers 18 a and 18 b have contact pieces facing towards each other thatmay be constituted by inwardly-directed convex surfaces of the curvedend portions 20 a and 20 b of the fingers 18 a and 18 b.

In the position of the measuring fingers 18 a and 18 b that enablesmeasurements to be performed, as shown in FIG. 2, the contact surfacesof the fingers bear against two zones of the outside surface of the rodthat are situated on two diametrically opposite generator lines.Generally, the surfaces of the contact pieces through which the fingerspress against the rod 2 are placed in diametrically opposite locationson a circular cross-section of the rod.

The support 13 and the measuring tool 14 are then caused to move in thevertical direction Z using the displacement arrangement associated withthe vertical direction displacement post 12.

The segment of rod extending between the spacer grids 3 is scannedeither by moving upwards from the lower grid to the upper grid, or bymoving downwards from the upper grid to the lower grid, said movementtaking place at a speed that is slow, and during said movement thediameter of the rod is measured continuously. During this displacementin the vertical direction, the measuring head 18 of the tool 14 isguided by the pulley wheels 15 a and 15 b whose grooves make contactwith et rod 2.

The results are displayed on the screen of the microcomputer 33 and/orprinted on the printer 34 in the form of diameter values expressed as afunction of time or as a function of the position of the measuringpoints in the vertical direction.

The vertical positions of the points may be identified, e.g. relative tothe bottom surface of the top nozzle 5 of the fuel assembly.

After measurements have been performed on one peripheral rod 2, it ispossible to perform measurements on an adjacent rod or on any otherperipheral rod situated in the face of the fuel assembly that isdirected towards the measuring head. After reversing the measuring headin the direction Y, it is moved in the direction X through theappropriate distance to pass from one peripheral rod to the next rodthat is to be measured.

In order to perform measurements on another face of the fuel assembly 1or in order to perform measurements on the diameters of peripheral rodsin another fuel assembly, the fuel assembly 1 is raised using thehandling arrangement carried by the bridge and the fuel assembly isturned about its axis so as to bring the face in which measurements areto be performed so as to be directed towards the measuring tool, orindeed a new fuel assembly is taken up and introduced in part into theelevator 7.

The method and the apparatus of the invention may be implemented toinspect the wear of a fuel rod 2 in a support zone of the rod situatedinside a spacer grid 3.

Initially, after the fuel assembly has been placed in the elevator orbefore the fuel assembly has been placed in the measuring position,traction is applied in the axial direction on the rod 2 of the fuelassembly so as to move its wear zone inside a cell of the spacer grid 3and bring said wear zone into a position that is situated above thespacer grid 3.

FIG. 4 illustrates a fuel rod 2 having a wear zone 35 due to the rodcoming into contact with a dimple inside a cell of the spacer grid 3,which wear zone is illustrated in its position after the rod 2 has beenmoved in the axial direction so that the wear zone 35 lies in a segmentof the rod situated above the spacer grid 3.

The measuring method is implemented using measuring fingers in an offsetdisposition enabling measurements to be performed in a zone situated inthe immediate vicinity of the top edge of the spacer grid 3. The guidepulley wheel(s) may be in the retracted position so as to make itpossible to get as close as possible to the grid. It is not possible tomove the fuel rod 2 of a fuel assembly in the vertical direction througha great amplitude because of the small clearance between the topportions of the rods and the bottom surface of the top nozzle.Furthermore, and as may be seen in FIG. 4, the spacer grid 3 has vanesprojecting from its top edge.

As a result, the measuring fingers of the measuring head 18 have endportions 20 a and 20 b constituted by feelers for making contact withthe fuel rod that are of special shape and disposition.

One of the feelers 20 a, referred to as a measuring feeler, has a tipenabling contact to be made between the feeler of the measuring fingerand the outside surface of the fuel rod 2 in the bottom of the wear zone35.

The other feeler 20 b referred to as a reference feeler, has arelatively large area without interruption so as to enable the measuringhead to bear effectively against the rod in a zone that is axial offsetrelative to the contact zone of the feeler 20 a.

After the wear zone 35 of the rod has been extracted from the spacergrid, and after calibration, the measuring head is put into place andmoved vertically so as to scan along a generator line 2 a of the outsidesurface of the rod 2 along a segment that includes the wear zone 35,immediately above the top portion of the spacer grid 3.

The feeler 20 b of the second measuring finger 18 b remains in contactwith a generator line 2 b of the rod 2 that is diametrically oppositethe generator line 2 a along which the tip of the feeler 20 a of thefirst measuring finger 18 a travels.

This makes it possible to measure very accurately the wear profile ofthe rod in the zone of the rod that has been retracted from the spacergrid.

The wear profile may be measured in two stages and between the stagesthe positions of the measuring feeler and of the reference feeler areinterchanged to act on opposite generator lines. This provides two wearprofiles for the rod.

The method and the device of the invention thus make it possible toperform very accurate dimensional examination on the fuel rods in anirradiated fuel assembly placed in the deactivation pool of a nuclearpower station.

The information provided by the measurements may be stored and indexedin the memory of the microcomputer.

The measuring method of the invention may be fully automated.

The invention is not limited specifically to the embodiment describedabove.

Thus, the measuring tool may be of a shape that is different from thatdescribed, and the displacement arrangement or the return arrangementacting on the various elements of the measuring tool may likewise beimplemented in other ways.

The invention applies not only to fuel assemblies for a nuclear reactorcooled by pressurized water, but also to other types of fuel assemblyhaving rods disposed in bundles and accessible from the side faces ofthe fuel assembly.

1. A method of measuring a diameter of a peripheral rod of a fuelassembly of a nuclear reactor cooled by light water, the fuel assemblycomprising a bundle of fuel rods and a framework for supporting the fuelrods, the framework comprising spacer grids distributed in an axiallongitudinal direction of the bundle of rods, guide tubes disposed incertain positions of the rods inside the bundle, and end nozzles, themeasuring being performed underwater in a pool in which the fuelassembly is placed so that the fuel assembly longitudinal axis extendsvertically, and being performed between two successive spacer gridsalong a longitudinal segment of a rod situated at a periphery of thefuel assembly, the method comprising the steps of: placing a measuringtool at a level situated between the two successive spacer grids, thetool having two measuring fingers with freedom to move relative to eachother in a first horizontal direction, the fingers being urged towardseach other by a resilient return arrangement and extending in a secondhorizontal direction perpendicular to the first horizontal direction;moving the measuring fingers in the second horizontal direction towardsthe rod so as to put two contact pieces into contact with two zones ofthe segment of rod occupying diametrically opposite generator lines, thecontact pieces being secured to respective ones of the measuringfingers; moving the two measuring fingers such that the contact piecesare pressed against the rod in two zones situated on the diametricallyopposite generator lines, so that the fingers travel in a vertical axialdirection of the rod along the segment situated between the two spacergrids; and using an electromagnetic sensor to measure a distance betweenthe contact pieces of the measuring fingers in the first horizontaldirection perpendicular to an axial displacement of the measuringfingers.
 2. The method according to claim 1, further comprising:calibrating the electromagnetic sensor by putting the contact pieces ofthe measuring fingers against the diametrically opposite generator linesof a reference rod prior to putting the measuring tool into place. 3.The method according to claim 1, further comprising measuring atemperature of the pool in a vicinity of the segment of fuel rod onwhich diameter measurements are performed; and correcting a spacingmeasured between the contact pieces of the measuring device as afunction of the temperature measurement.
 4. A method of measuring thediameter of a peripheral rod of the fuel assembly according to claim 1,along a segment of the peripheral fuel rod that is engaged forsupporting the rod in a cell of the spacer grid and having a wear zonein an outer surface, the method comprising: initially applying tractionto the fuel rod to cause the rod to move axially relative to the spacergrid so as to extract the segment of fuel rod that includes the wearzone from the spacer grid; and implementing the measuring method on thesegment of rod that includes the wear zone extracted from the spacergrid by moving the measuring fingers which has the contact pieces incontact with the diametrically opposite generator lines of the outersurface of the rod in the axial direction along the length of thesegment that includes the wear zone.
 5. A device for measuring adiameter of a peripheral rod of a fuel assembly of a nuclear reactorcooled by light water, the assembly comprising a bundle of fuel rods anda framework for supporting the fuel rods, the framework comprisingspacer grids distributed in an axial longitudinal direction of thebundle of rods, guide tubes placed in certain positions for rods insidethe bundle, and end nozzles, measurement taking place underwater in apool in which the fuel assembly is placed with its longitudinal axis ina vertical direction, and being performed between two successive spacergrids along a longitudinal segment of the rod situated at the peripheryof the fuel assembly, the device comprising: a measuring tool comprisingtwo measuring fingers mounted on a tool support with freedom to moverelative to each other in a first horizontal direction, the fingersbeing urged towards each other by a resilient return arrangement andextending in a second horizontal direction perpendicular to the firsthorizontal direction; an electromagnetic displacement sensor connectedto the measuring fingers via two respective portions that are movablerelative to each other; an arrangement for moving the measuring tool inthe two horizontal directions and in a vertical direction; and ameasuring station having an arrangement for receiving and processingmeasurement signals from the electromagnetic sensor.
 6. The deviceaccording to claim 5, wherein the arrangement for moving the measuringtool in the first and second horizontal directions is a set of X-Ydisplacement tables.
 7. The device according to claim 5, wherein thearrangement for moving the measuring tool in the vertical direction is aguide post and an arrangement associated with the guide post forimplementing displacement at a slow speed.
 8. The device according toclaim 5, wherein the electromagnetic sensor for measuring a distancebetween the contact pieces of the measuring fingers is an LVDT sensorcomprising a body with an electrical coil and a plunger core secured toa rod, the body of the sensor being fixed to a first of the twomeasuring fingers and the rod of the sensor being fixed to a second ofthe two measuring fingers.
 9. The device according to claim 5, whereinthe measuring station placed above a level of the pool comprises: a unitfor acquiring and processing analog signals from the electromagneticsensor; and a computer for making use of the signals.
 10. A deviceaccording to claim 5, further comprising: an arrangement for viewing thefuel assembly and the measuring device in the pool; and an arrangementconfigured to control the displacements of the measuring tool.
 11. Adevice according to claim 10, wherein the arrangement for viewing themeasuring tool comprises video cameras configured to further view thefingers of the measuring tool for aiming at a peripheral rod of the fuelassembly.
 12. A device according to claim 5, further comprising: a probeto measure a temperature in the pool in a vicinity of the measuringtool.